The "Connect via…" button allows you to select Telenet or Tymnet. The "Modem String…" button allows you to enter an optional modem command to send to your modem before it dials. Click on this button to enter a modem string.
AppleLink first tries to connect using Network Phone #1, and the first "Connect via…" file. If it fails it will try again using Network Phone #2, and the seccond "Connect via…" file.